1. Field
Apparatuses and methods consistent with the present invention relate to detection systems, including the detection of fluorescence, absorbance, and chemiluminescence in samples placed in the wells of microplates.
2. Description of the Related Work
Multiple analytical instruments are used in laboratories to evaluate samples under test that are placed into vessels of various shapes. In the past twenty years, a microplate format has become very popular, as it lends itself to testing many samples on a single matrix-style receptacle. The first detection systems for microplates were absorbance readers. Later dedicated fluorometers were developed, followed by instruments to measure chemiluminescence.
The range of assay chemistries and labeling technologies continues to grow. Currently employed detection methods include absorbance, multiplexed fluorescence and chemiluminescence, fluorescence polarization (FP), time-resolved fluorescence (TRF), fluorescence resonance energy transfer (FRET), quenching methods, and specially designed labels with intensity and spectral responsiveness to environmental conditions. Along with this range of detection methods, users are conjugating an ever-growing array of organic and inorganic labels for targets, ranging from small-molecule drug candidates to proteins and nucleic acids, and to subcellular structures and cells.
FIG. 1 illustrates the general structure of a related art multimode detection system. As shown in FIG. 1, a typical system comprises a light source 10, an excitation spectral device 20, an optical module 30, a measurement chamber 60 with samples 70, an emission spectral device 40, and a detector 50. There are two distinct types of related art multimode detection systems: filter-based units and monochromator-based units.
Filter-based units, when offered with high quality filters in combination with dichroic mirrors, allow for measurements with very low detection limits. This is mainly due to a high signal level, which is achieved with the filters, in combination with a high signal-to-noise ratio, which is achieved by a high level of blocking of the unwanted radiation around the desired waveband. The transmittance of filters is routinely over 50%, and this high level of transmittance can be achieved independent of the wavelength. Therefore, a very broad spectral range can be covered equally well from the deep ultraviolet (UV) to the infrared (IR), and the bandpass of the filter can be tailored to the specific application.
However, the filter-based unit cannot obtain a spectral scan for excitation or emission of the substance under investigation. A user must know upfront what substance he or she is working with and order an appropriate filter set. In addition, when working in the deep UV, filters tend to degrade when exposed to the UV radiation of the light source, due to solarization. Also, maintaining libraries of filters for the full range of labels is prohibitively expensive, and appropriate combinations are often not readily available for a given label, conjugation chemistry, target molecule, and assay condition. Further, the effects of these conditions are not always predictable based on the nominal spectra of the basic label.
Monochromator-based instruments offer a high level of flexibility in terms of choosing the wavelengths and obtaining scans of excitation and emission spectra, thus allowing the user to work with unknown substances. This also permits optimization of the measurements for perturbations to the spectra of labels due to assay conditions, conjugation chemistries, and target molecules. Additionally, when working with real biological or biochemical samples, interfering signals from other sample components may require optimization of excitation and emission wavelengths for the exact assay conditions.
The monochromators used in modern instruments are usually based on diffraction gratings, and use a flat grating for dispersion and concave mirrors for focusing light, or concave gratings that combine dispersive and focusing functions. Monochromators require order sorting filters to separate high spectral orders, but in the range from 200 nm to about 380 nm, no order sorting filters are needed. Therefore, there is no need for filters that withstand UV radiation, and the solarization problem is avoided.
However, the response of the monochromator is not constant across the wavelength range. One can obtain a system with a good signal in the UV, the visible, or the IR; however, one cannot obtain a system with a good signal in all of the wavelength ranges in the same monochromator-based unit. A usual compromise is to optimize the excitation monochromator in the UV and to optimize the emission monochromator in the visible or IR, because the wavelength of the emission light shifts to the right with respect to the wavelength of the excitation light.
In order to obtain low detection limits, the monochromator must have very low stray light. A traditional way to achieve this in the monochromator-based system is to employ two stage monochromators. These are called double monochromators, and contain two single monochromators placed in series. While this does result in very low stray light, the penalty is a dramatic decrease in signal, especially in spectral regions where the response of the single stage monochromator is already low. There are several instruments in the field based on this method, such as the Tecan INFINITE and the Thermo VARIOSKAN.
In terms of performance, the filter-based units achieve significantly lower detection limits in fluorescence intensity applications across the full spectral range, and work significantly better with techniques such as TRF, FP, and Homogeneous Time-Resolved Fluorescence (HTRF), all of which require the strong signal provided by the filter-based units. On the other hand, the monochromator-based units provide the flexibility of choosing any wavelength and the ability to obtain a spectral scan, at the expense of lower sensitivity.
U.S. Pat. No. 6,313,471 describes a method that combines bandpass filters and monochromators in series in a detection system. In this method, the bandpass filter acts as a crude first stage monochromator. The instrument splits the full spectral range of interest into several regions corresponding to the number of filters employed, and blocks radiation from adjacent regions by using additional filters. The single stage monochromator that follows the bandpass filters then selects the wavelength of interest from this prefiltered range.
However, with a limited number of prefiltered regions, this method is limited in flexibility. If both the excitation and emission wavelengths fall into one region, the method is not effective in achieving low stray light or high performance. True spectral scanning is not readily accomplished with this method. This limits its utility for spectral measurement and optimization under conditions of fine spectral perturbation.
There is currently no related art instrument that combines the best of filter-based measurements and monochromator-based measurements in a single unit, and allows a user to choose between the two measurement types. A need exists for a single multidetection system that combines the low detection limits and assay methodology flexibility of filter-based units with the wavelength flexibility and superior UV performance of the monochromator-based units. This instrument could provide a user with a full flexibility in terms of assay type and wavelength selection.